Experimental study on the comprehensive heat transfer performance of double-pipe heat exchangers with twisted elliptical tubes and with corrugated tubes

Twisted elliptical tubes (TETs) and corrugated tubes (CTs) are both key elements for enhancing heat transfer in double-pipe heat exchangers (DPHEs). In order to investigate the comprehensive heat transfer performance of these DPHEs, an experimental study was conducted. The Nusselt number ($Nu$) and friction factor ($f$) criterion correlation formulas for the tube side and shell side of the two DPHEs were fitted, and a factor ($\eta$) was used to evaluate the comprehensive heat transfer performance. Experimental results indicated that both the overall heat transfer coefficients ($U$) of the twisted elliptical tube double-pipe heat exchanger (TETDPHE) and the corrugated tube double-pipe heat exchanger (CTDPHE) increased as the flow velocity ranged from 0.1 to 1.5 m/s, and the latter had an average increase of 13 % compared to the former. Under the same Reynolds number ($Re$), the $U$ and pressure drop ($\Delta P$) of these two DPHEs were basically not affected by the temperature difference ($\Delta T$) between the hot and cool side at the inlet. The maximum $\eta$ values of TETDPHE and CTDPHE were 1.43 times and 1.24 times of the smooth circular tube double-pipe heat exchanger (SCTDPHE), respectively. In the range of $Nu$ from 2000 to 31000, the $\eta$ value of TETDPHE was better than that of CTDPHE and SCTDPHE. In the range of $Nu$ from 2000 to 8300, the $\eta$ value of CTDPHE was better than that of SCTDPHE. Thus, the CTDPHE is suitable for the scenario with lower Reynolds numbers, while the TETDPHE can be widely applied.